In recent years, magnetoresistive devices that utilize tunnel magnetoresistive effects are actively and experimentally used in magnetic random access memories.
Where a magnetoresistive device is used as a memory device in a magnetic random access memory, the magnetoresistive device normally has a ferromagnetic single tunnel junction or a ferromagnetic double tunnel junction. In a ferromagnetic single tunnel junction, one of the two ferromagnetic layers sandwiching a tunnel barrier layer is a fixed magnetization layer (a magnetization reference layer) that has a fixed magnetization direction, and the other one of the two ferromagnetic layers is a magnetic recording layer that has a variable magnetization direction. A ferromagnetic double tunnel junction has a stacked structure formed with a first magnetization reference layer, a first tunnel barrier layer, a magnetic recording layer, a second tunnel barrier layer, and a second magnetization reference layer. Hereinafter, a ferromagnetic single tunnel junction or a ferromagnetic double tunnel junction will also be referred to simply as a MTJ. A memory device having such a MTJ is capable of storing data in a nonvolatile manner, and characteristically requires a short writing time or a short reading time of 10 ns or less. Also, such a memory device is characteristically capable of performing rewriting 1015 times or more.
As a method of performing writing on such a magnetoresistive device, a spin-injection magnetization reversing technique has been suggested (see U.S. Pat. No. 6,256,223, for example). By the spin-injection magnetization reversing technique, the magnetization of a magnetic recording layer is reversed by injecting spin-polarized electrons (spin injection current) into a magnetic recording layer of a memory device. The value of the spin injection current required for writing becomes smaller as the magnetic recording layer becomes smaller. Accordingly, there is a high expectation for the spin-injection magnetization reversing technique as a method of performing writing on a magnetic random access memory.
However, to perform magnetic writing by the spin-injection magnetization reversing technique, it is necessary to cause a bidirectional current to flow in a magnetoresistive device. Therefore, diode-type architecture (so-called cross-point architecture) that connects magnetoresistive devices and diodes in series between bit lines and word lines cannot be used. In view of this, architecture in which each one memory cell is formed with at least one transistor and one magnetoresistive device needs to be employed, and, as a result, the cell size cannot be made equal to or smaller than the cell size of a dynamic random access memory (DRAM).
As a memory device of a spin-injection magnetization reversal type, there is a known spin transistor (hereinafter also referred to as a spin MOS transistor) having a MTJ in at least either a source electrode or a drain electrode (see Japanese Laid-Open Patent Publication No. 2008-66596, for example). Since a bidirectional current is required at the time of writing in this spin transistor, a switching transistor for switching the direction of the current needs to be connected to each of the source and drain electrodes of the spin transistor. Therefore, the problem of an increased total circuit area is caused as in a magnetoresistive device that performs magnetic writing with spin-injection magnetization reversals.
As described above, a bidirectional current is required for writing in a spin memory or a spin transistor of a conventional spin-injection magnetization reversal type, and the total circuit area cannot be made smaller.